Epoxy resins have generally been employed in a variety of applications requiring high performance materials. Cure of an epoxy resin can generally be achieved by two package systems based on the incorporation into the resin of active amine-containing compounds or carboxylic acid anhydrides. These systems require thorough mixing of the ingredients; in addition, cure time can be several hours.
Another catalyst which can be used to cure epoxy resins as "one package" systems is based on the employment of a Bronsted Acid catalyst in the form of an amine complex such as boron trifluoride-monethyl amine. The Bronsted Acid is released on heating; cure takes place within 1 to 8 hours and can require a temperature of 160.degree. C. and higher. As a result, these one package epoxy compositions cannot be employed to coat heat sensitive devices such as delicate electronic components. Nor can epoxy monomers having low boiling points be used due to the resulting losses to evaporation during cure.
As shown by Schlesinger, U.S. Pat. No. 3,703,296, certain photosensitive aromatic diazonium salts can be employed to cure epoxy resins. When photolyzed, these aromatic diazonium salts are capable of releasing, in situ, a Bronsted Acid catalyst which can initiate the rapid polymerization of the epoxy resin. However, even though these one package epoxy resin mixtures can provide fast curing compositions, a stabilizer must be used to minimize cure in the dark during storage of these mixtures. Despite the measures, gellation of the mixture can occur even in the absence of light. In addition, nitrogen is released during UV-cure, which can result in film imperfections. Diazonium salts are generally thermally unstable, rendering the use of such materials hazardous because of the possibility of run-away decomposition.
U.S. Pat. No. 4,138,255 of Crivello describes yet another catalyst system. It employs radiation sensitive aromatic onium salts of the formula: EQU [(R).sub.a (R.sup.1).sub.b (R.sup.2).sub.c X].sub.d.sup.+ [MQ.sub.e ]--(e--f)
where R is a monovalent aromatic organic radical, R.sup.1 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl; R.sup.2 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals, X is a Group VIa element selected from sulfur, selenium and tellurium, M is a metal or metalloid, Q is a halogen radical, a is a whole number equal to 0 to 3 inclusive, b is a whole number equal to 0 to 2 inclusive, c is a whole number equal to 0 to 1, where the sum of a+b+c is a value equal to 3 or the valence of X, EQU d=e-f
f=valence of M and is an integer equal to from 2 to 7 inclusive PA0 e is&gt;f and is an integer having a value up to 8. These catalysts are ordinarily activated by radiant energy such as an electron beam or ultraviolet light. PA0 each R is a monovalent organic aromatic radical; PA0 X is selected from the group consisting of sulfur and iodine; PA0 C is a Bronsted Acid polymerization catalyst precursor; PA0 a equals the absolute value of (the valence number of X minus 1); and PA0 b equals the absolute value of the valence number of C. It is combined with an amount of a peroxide compound effective to activate said polymerization initiator.
Notwithstanding these available systems, additional polymerization initiators have been sought. These include initiators which are thermally activatable and, in particularly, which are activated only at elevated temperatures so as to be stable under ambient conditions.
These and yet additional objectives or advantages are obtained in accordance with the present invention.